We will assess covariation between individuals' sensitivity to binocular stimuli both a) to model mechanisms underlying binocular vision and b) to evaluate the influence of genetic vs. experiential factors upon sensitivity to binocular depth. a) Model binocular vision mechanisms using covariation between individuals. We will assess how distinct the classes of mechanisms are that process 1) depths closer ("crossed") vs. further ("uncrossed") than fixation, and 2) short-duration ("transient") vs. long-duration ("sustained") depth stimuli. We will also 3) determine whether a common mechanism may process vertical disparity for both perception of depth and control of vergence eye posture. To do this, we will use classical covariation based modeling methods that have been used in the study of color vision, contrast sensitivity, and visual development, and that I have extended in my dissertation. These methods have not previously been applied to the study of binocular vision. b) Determine the effects of genes vs. experience on sensitivity to binocular depth. Richards (1970) showed preliminary evidence that sensitivity to transient depth is a heritable trait. We will conduct a study of identical ("monozygotic") and fraternal ("dizygotic") twins to more rigorously establish the influence of genes vs. experience upon such sensitivity.